1. Field of the Invention
The present invention relates to a tetrafluoroethylene copolymer resin powder composition capable of providing a highly corrosion- and chemical-resistant coating.
It further relates to a tetrafluoroethylene copolymer resin powder composition suitable for rotational lining processes to generate a thick coating which is essentially free of gas bubbles and which has strong adhesion to, and peel resistance from, a substrate.
2. Background
A copolymer (sometimes referred to hereinafter as PFA) obtained by copolymerizing tetrafluoroethylene (sometimes referred to hereinafter as TFE) with a perfluoro(alkyl vinyl ether) (sometimes referred to hereinafter as PFVE) is superior to other fluorinated polymers, particularly to polyvinylidene fluoride and ethylene/tetrafluoroethylene copolymers, with respect to heat resistance, chemical resistance, and electrical properties. PFA is different from polytetrafluoroethylene (sometimes referred to hereinafter as PTFE), a tetrafluoroethylene homopolymer, in that PFA is melt flowable at its melting point or above, so that it is extensively used as an excellent pinhole-free, or void-free, film-forming material.
Such film-forming material can be used to provide a substrate surface with corrosion resistance, nonstickiness, wear resistance, and chemical resistance. Such films are used over a broad temperature range, from low to high temperatures. Methods are known in the art for generating a corrosion-resistant film. Such methods include powder coating, sheet lining, and the like.
Generating a thick corrosion-resistant film by powder coating requires applying a superposed plurality of relatively thin films, each about 100 .mu.m thick, because an attempt to generate such a thick film by a single application tends to result in a film which suffers from bubbling. Even a multiplicity of forming steps permits generating a film thickness of at most only about 1 mm, and frequently bubbling occurs.
Sheet lining is a method for applying pieces of 2-5 mm thick PFA or PTFE sheet, together with glass cloth backing, to a substrate for bonding by an adhesive, where the pieces are bonded together by welding. The resulting film, although thick, can only be used at temperatures lower than those which can be tolerated by the PFA or PTFE, because of the heat resistance of the adhesive used.
A rotational lining method has recently attracted attention as a way of overcoming the above deficiency by generating a thick film lining, 1-5 mm in thickness, by a single resin melting operation. Rotational lining is a method of application which calls for heating a rotating mold on which a film is to be generated, introducing an appropriate melt flowable resin powder into the mold, melt fusing the resin on the interior surface of the mold so that the resin reaches a given thickness and sticks to the mold. Objects so formed can be used for lined tanks, pipes, and the like.
However, a rotational lining method using PFA has not been practical. Because the resin shrinks more than the metal substrate used, the resin, once heat-fused to the substrate, develops an internal stress, resulting in a coating which peels off spontaneously soon after the formation of the coating or after standing for several days, or which peels off when subjected to several heating-cooling cycles. This kind of peeling cannot be prevented sufficiently by conditioning the substrate by blasting or by primer coating, and the problem worsens as the thickness of the coating increases.
Powder coating comprises adding a heat resistant filler powder into a resin layer and generating a relatively thin resin coated layer, 50-500 .mu.m thick, which retards shrinkage and prevents peeling (Japanese Patent Application Publication 54-3172). However, a powder coating composition for generating the above thin film still does not exhibit satisfactory peel resistance when applied as a rotational lining for generating thick films, and fails to give a uniform lining coating because the powder composition bubbles or has poor melt flow.
PFA, near the molding temperature of 340.degree. C.-380.degree. C., tends to suffer from considerable bubbling, particularly if a thick film, and also fails to yield satisfactory coated films in rotational lining applications because of the bubbling phenomenon. The bubbling may occur because of the retention of gas generated by the thermal decomposition of unstable PFA end groups or the retention of gas due to insufficient deaeration of the powder and filler used. Rotational molding, in which the molded resin article formed by melt molding is removed from the mold, also suffers from a bubbling problem, but the bubbling can be prevented by treating the PFA resin powder with fluorine, thereby stabilizing any unstable end groups (Kokai 60-240713). However, a lining composition which contains a peel-preventing filler cannot be subjected to the fluorination treatment because the filler is attacked by fluorine. The bubbling of a coating, which reduces the essential thickness of the coating, considerably deteriorates the corrosion resistance of the lining.
Methods are known in the art for solving PFA bubbling in the case of coating or powder coating. These methods include utilizing heat stabilizers such as zinc or tin, or organic sulfur compounds such as benzoimidazole type mercaptans, and the like, thereby preventing the copolymer from bubbling (Japanese Patent Application Publication 57-14774). However, the use of zinc as a heat stabilizer for a PFA lining coating causes the film to crack, because zinc tends to be attacked by acids or alkalis, thereby allowing chemicals to penetrate. Such material is unsatisfactory as a corrosion resistant lining material for semiconductor manufacturing processes which cannot tolerate any metal-leaching contamination.
A process for decreasing the amount of zinc and using polyphenylene sulfide (hereafter PPS) at the same time (Japanese Patent Application Publication 63-38065), does not solve either the metal-leaching or the bubbling problem.